The Pac-Man Dossier

[What design and AI lessons can we learn from Namco's seminal Pac-Man? From history through behavior, Gamasutra presents a comprehensive Jamey Pittman-authored guide to the classic game.]

In
1999, Billy Mitchell of Hollywood, Florida became the first person
to obtain a perfect score of 3,333,360 at Pac-Man, eating
every possible dot, energizer, ghost, and bonus on every level
without losing a single life in the process.

But perhaps what is
most amazing is the fact he can play without using any memorized
routines widely known as "patterns".

Instead, he
relies on his familiarity with how each ghost behaves as it moves
through the maze, using that knowledge to keep Pac-Man one step
ahead of his enemies at all times.

Unlike
Mitchell, most players are only able to rack up high scores with
the aid of multiple patterns that take advantage of the game's
deterministic nature.

These patterns require perfect memorization
and recall to be of any real use - a single hesitation or
wrong turn during execution can make the remainder of a pattern
useless.

Not surprisingly, an over-reliance on these routines
leaves many a player clueless as to how to effectively avoid the
ghosts and finish off the remaining dots in the higher levels once
a mistake occurs.

Most
Pac-Man strategy guides available on the internets today
are very similar in content to the books that used to be sold back
in the early 80s

A summary of gameplay and scoring is provided
first, followed by a list of patterns to be memorized by the
reader, but very little insight is offered on how the game works
or how the ghosts make decisions.

Therefore, the purpose of this
guide is to give the player and game designers a better design understanding of Pac-Man by taking a closer look at gameplay,
maze logic, ghost personalities, and the mysterious "split
screen" level.

All
information provided has been extracted from or verified with the
disassembly output from the Midway Pac-Man
arcade ROMs along
with controlled observations of actual gameplay. As such, I have a
high confidence in its accuracy.

That said, if you notice an error
or omission, please contact me so it can be corrected as soon as possible. I hope you find the
information just as interesting and useful as I did for gaining a
better understanding of this classic game.

Special
thanks to Don Hodges (www.donhodges.com)
whose invaluable contributions to this guide can be found in every
chapter.

Chapter 1: Welcome to the Machine

"I
don't have any particular interest in [computers]. I'm interested
in creating images that communicate with people. A computer is not
the only medium that uses images; I could use the movies or
television or any other visual medium. It just so happens I use
the computer."-Toru Iwatani

It was 1977 when a
self-taught, capable young man named Toru Iwatani came to work for
Namco Limited, a Tokyo-based amusement manufacturer whose main
product lines at the time were projection-based amusement rides
and light gun shooting galleries.

He was just 22 years old with no
formal training in computers, visual arts, or graphic design, but
his creativity and aptitude for game design were obvious to the
Namco executives that met with Iwatani. They offered to hire
him-with assurances they would find a place for him in the
company-and he accepted.

Iwatani
eventually found his place designing titles for Namco's new video
games division. His limited computer skills necessitated his being
paired with a programmer who would write the actual code while
Iwatani took on the role of game designer for the project.

This
was a new job for the game industry in 1977 when most games were
designed by the programmers who coded them. In addition to a
programmer, Iwatani's team would usually include a hardware
engineer to develop the various devices and components, a graphic
artist to realize his visual ideas, and a music composer for any
music and sound effects needed in the game.

Iwatani
had initially wanted to work on pinball machines, but Namco had no
interest in the pinball business. Perhaps as a concession, his
first game design, called Gee Bee, was
a paddle game similar to Atari's Breakout
but with a decidedly
pinball-inspired slant to the gameplay.

Released in 1978, it was
Namco's first original video game-they had only ported
existing Atari games to the Japanese market up to this point-and
it enjoyed moderate success in the arcades.

But
the paddle games were losing ground fast to a new genre. The
unprecedented success of Taito's Space Invaders in
1978 caused an industry-wide shift toward space-themed,
shoot-'em-up games (as well as a national coin shortage in Japan).

Game manufacturers scrambled to match Taito's success with space
shooters of their own. Namco was quick to follow suit, assigning a
team to start work on a Space Invaders clone
at once. It was around this time that Toru Iwatani began thinking
about designing a different kind of game. He felt the shoot-'em-up
craze was destined to fade away like the paddle games before them.

Rather than make another space shooter, Toru wanted to take his
game design in a completely new direction that did not focus on
violence or conflict, and would appeal to both male and
female audiences.

He
took inspiration from a children's story about a creature that
protected children from monsters by eating them. One of Iwatani's
design methods included taking key words associated with a story
to aid in developing his ideas. The kanji word taberu
("to eat"),
became the premise for the game.

The word kuchi
("mouth") has a
square shape for its kanji symbol and provided the inspiration for
the game's main character-the better-known legend of Iwatani
receiving his inspiration from a pizza pie with a slice missing
was, by his own admission, not entirely correct:

"Well,
it's half true. In Japanese the character for mouth (kuchi) is a
square shape. It's not circular like the pizza, but I decided to
round it out. There was the temptation to make the Pac-Man shape
less simple. While I was designing this game, someone suggested we
add eyes. But we eventually discarded that idea because once we
added eyes, we would want to add glasses and maybe a moustache. There would just be no end to it.

Food is the other part of the
basic concept. In my initial design, I had put the player in the
midst of food all over the screen. As I thought about it, I
realized the player wouldn't know exactly what to do: the purpose
of the game would be obscure. So I created a maze and put the food
in it. Then whoever played the game would have some structure by
moving through the maze.

The Japanese have a slang word-paku
paku-they use to describe the motion of the mouth opening
and closing while one eats. The name Puck-Man came from that
word."

-Toru
Iwatani

The monsters from
the children's story were included as four ghosts that chase the
player through the maze, providing an element of tension. Attacks
on the player were designed to come in waves (similar to Space
Invaders) as opposed to an endless assault, and each ghost was
given an unique personality and character.

The children's story
also included the concept of kokoro ("spirit")
or a life force used by the creature that allowed him to eat the
monsters. Toru incorporated this aspect of the story as four
edible power pellets in the maze that turn the tables on the
ghosts, making them vulnerable to being eaten by the player.

With
a name and a basic design in place, Iwatani was ready to begin
work. The team Namco assigned Iwatani to bring Puck-Man to
life included a programmer (Shigeo Funaki), a hardware engineer, a
cabinet designer, and a music composer (Toshio Kai).

Development
got underway in early 1979. In the course of that year, two new
pinball-themed designs from Iwatani-Bomb Bee and
Cutie Q-were both released during Puck-Man's
development cycle. Both games were similar to Gee Bee but
with stronger gameplay and improved visuals.

The Namco team
working on the Space Invaders clone for the past several
months had just achieved a technological coup for Namco: the first
game to use a true, multi-colored, RGB display instead of the
monochrome monitors with colored cellophane tape so prevalent at
the time.

Thanks to the breakthrough of the other team, Iwatani
now had the new promise of color to enhance his design. Mindful
that he wanted the game to appeal to women, he immediately decided
to use it on the ghosts, choosing pastel shades for the bodies and
adding expressive, blue eyes. Dark blue was used for the maze
itself, while Puck-Man was drenched in a brilliant yellow.

The
look and feel of Puck-Man continued to evolve for over a
year. A large amount of time and effort was put into developing
the ghosts unique movement patterns through the maze and tweaking
the game difficulty variables as boards were cleared.

Bonus
symbols (including the Galaxian flagship) were added into
the mix at some point, and the ghosts were finally given names:
Akabei, Pinky, Aosuke, and Guzuta. Sound effects and music were
some of the final touches added as development neared an end along
with constant tweaking of the ghosts' behavior.

Puck-Man's
creation was a year and five months in the making-the
longest ever for a video game to that point. Finally, on May 22nd,
1980, it was released to arcades in Japan. Initially, the game did
moderately well, but was no overnight sensation.

In fact, Namco's
multi-colored Space Invaders clone, called Galaxian,
was much more popular with the gaming public-the
predominately male, game-playing audience in Japan was unsure what
to make of Puck-Man with its cartoon-like characters, maze,
and pastel colors, whereas Galaxian was more immediately
familiar to them with its shoot-'em-up space theme.

Midway
was a distributor of coin-operated video games in the U.S. that
was always looking for the next big hit from Japan to license and
bring to America. They opted for both Puck-Man and
Galaxian, modifying
the cabinets and artwork to make them easier to manufacture as
well as providing a more American look and feel.

Puck-Man
went through the majority of the
changes: the cabinet was modified slightly, changing the color
from white to a bright yellow to make it stand out in the arcade.
The detailed, multi-colored cabinet artwork was replaced with
cheaper-to-produce, three-color artwork illustrating an iconic
representation of Puck-Man (now drawn with eyes and feet) and one
blue ghost.

English names were given to the ghosts (Blinky, Pinky,
Inky, and Clyde), and the Namco title was changed to Midway. The
most significant change to Puck-Man was
the name. Midway feared it would be too easy for nasty-minded
vandals to change the P in Puck-Man to
an F, creating an unsavory epithet.

Not wanting their product
associated with this word, Midway renamed the game Pac-Man
before releasing it to American
arcades in October 1980.

But
the situation in America was reversed from Japan for these two
titles. Galaxian got
lost in the shuffle of the shoot-'em-up craze that blanketed
America's arcades and, by the fall of 1980, it was already
competing with more advanced video games like Defender.

In the end, Galaxian enjoyed
moderate success in America and in Japan, but was never the smash
hit the original Space Invaders was.
Pac-Man was another
story. There were no games to compare it to-it was in a
genre all by itself. The bright yellow cabinet, visuals, and
sounds drew a great deal of attention. No one had seen a game
quite like this before.

The addictive gameplay and challenge of
increasing levels of difficulty kept the die-hard gamers more than
happy, while the simplicity of the game appealed to younger
children. The lack of war-like motifs and violence did as Iwatani
had hoped and attracted a sizable female audience-a first
for a video game. Even the parents wary of the violence-themed
arcade games had no problem with their kids playing as cute and
innocuous a game as Pac-Man.

Pac-Man
went on to capture the world's imagination like nothing before or
since. It was a genuine phenomenon on a global scale, selling over
100,000 machines in its first year alone. Easy to learn but
notoriously difficult to master, everyone from school children to
Wall Street executives dropped quarter after quarter into an
ever-increasing number of waiting
Pac-Man machines.

A novelty song called "Pac-Man Fever"
received significant radio play, reaching number nine on the U.S.
Billboard charts. Many books were written offering tips and tricks
used by the best players to achieve high scores-the
first-ever strategy guides published for a video game.

Fast-forward to
nearly thirty years later: Pac-Man remains the best-selling
coin-operated video game in history. Still considered the most
widely-recognized video game character in the U.S., his likeness
has been licensed to over 250 companies for over 400 products.

His
namesake has been adopted by the business world to describe a way
to defend against a hostile takeover (the defending company
swallows up the larger company instead in a move known as the
"Pac-Man defense"). There is even an upright Pac-Man
machine on display at the
Smithsonian Institution in Washington, D.C.

Unlike the majority of
his early-80s contemporaries, new Pac-Man games
are still in development today. Most recently, Pac-Man
Championship Edition was
released in 2007 for the X-Box 360 console with the aid of Namco
game designer Toru Iwatani.

Interest
in the original coin-op title has never completely faded,
thankfully. Thanks to Namco's re-release of Pac-Man
and other arcade
classics for modern home consoles, new generations of Pac-addicts
have worn their hands out playing a game often older than
themselves.

Many classic titles are also kept alive thanks to the
advent of high-quality arcade emulators available for the home
computer (like MAME) that use a software copy of the arcade ROM
chips to recreate the game with 100% accuracy. Several web pages
with information about the original Pac-Man
arcade game can be
found online including Wikipedia
and the Killer
List Of Video Games.

Chapter 2: Gameplay Details

"As
Pac-Man was originally conceived to appeal to women players, it is
a very easy and approachable game. I believe that is an ingredient
in the longevity of the game."-Toru Iwatani, creator
of Pac-Man

The
Basics

The
premise of Pac-Man is delightfully simple: using a four-way
joystick, the player guides Pac-Man-up, down, left, and
right-through a maze filled with dots for him to gobble up.
Four ghost monsters are also in the maze and chase after our hero,
trying to capture and kill him.

The goal is to clear the maze of
dots while avoiding the deadly ghosts. Each round starts with the
ghosts in the "monster pen" at the center of the maze,
emerging from it to join in the chase.

If Pac-Man is captured by a
ghost, a life is lost, the ghosts are returned to their pen, and a
new Pac-Man is placed at the starting position before play
continues. When the maze is cleared of all dots, the board is
reset, and a new round begins. If Pac-Man gets caught by a ghost
when he has no extra lives, the game is over.

There are 244 dots
in the maze, and Pac-Man must eat them all in order to proceed to
the next round. The 240 small dots are worth ten points each, and
the four large, flashing dots - best known as energizers - are
worth 50 points each.

This yields a total of 2,600 points for
clearing the maze of dots each round. Players have two ways to
increase their score beyond what is earned from eating dots:

The first way to
increase your score each round is by turning the tables on your
enemies by making them your prey. Whenever Pac-Man eats one of the
four energizer dots located in the corners of the maze, the ghosts
reverse their direction and, in early levels, turn the same shade
of blue for a short period of time before returning to normal.

While blue, they are vulnerable to Pac-Man and can be gobbled up
for extra points providing they are caught before the time
expires. After being eaten, a ghost's eyes will return to the
monster pen where it is resurrected, exiting to chase Pac-Man once
again.

The first ghost captured after an energizer has been eaten
is always worth 200 points. Each additional ghost captured from
the same energizer will then be worth twice as many points as the
one before it-400, 800, and 1,600 points, respectively. If
all four ghosts are captured at all four energizers, an additional
12,000 points can be earned on these earlier levels. This should
not prove too terribly difficult to achieve for the first few
rounds as the ghosts initially remain blue for several seconds.

Soon after, however, the ghosts' "blue time" will get
reduced to one or two seconds at the most, making it much more
problematic to capture all four before time runs out on these
boards. By level 19, the ghosts stop turning blue altogether and
can no longer be eaten for additional points.

The second way to
increase your score each round is by eating the bonus
symbols (commonly
known as fruit)
that appear directly below the monster pen twice each round for
additional points. The first bonus fruit appears after 70 dots
have been cleared from the maze; the second one appears after 170
dots are cleared.

Each fruit is worth anywhere from 100 to 5,000
points, depending on what level the player is currently on.
Whenever a fruit appears, the amount of time it stays on the
screen before disappearing is always between nine and ten seconds.
The exact
duration (i.e., 9.3333 seconds,
10.0 seconds, 9.75 seconds, etc.) is variable and does
not become predictable with
the use of patterns. In other words, executing the same
pattern on the same
level twice is no guarantee for
how long the bonus fruit will stay onscreen each time.

This
usually goes unnoticed given that the majority of patterns are
designed to eat the bonus fruit as quickly as possible after it
has been triggered to appear. The symbols used for the last six
rounds completed, plus the current round are also shown along the
bottom edge of the screen (often called the fruit counter or level
counter). See Table
A.1 in the
appendices for all bonus fruit and scoring values, per level.

Ghosts have three
mutually-exclusive modes of behavior they can be in during play:
chase, scatter, and frightened. Each mode has
a different objective/goal to be carried out:

CHASE - A
ghost's objective in chase mode is to find and capture
Pac-Man by hunting him down through the maze. Each ghost exhibits
unique behavior when chasing Pac-Man, giving them their different
personalities: Blinky (red) is very aggressive and hard to shake
once he gets behind you, Pinky (pink) tends to get in front of
you and cut you off, Inky (light blue) is the least predictable
of the bunch, and Clyde (orange) seems to do his own thing and
stay out of the way.

SCATTER - In
scatter mode, the ghosts give up the chase for a few
seconds and head for their respective home corners. It is a
welcome but brief rest-soon enough, they will revert to
chase mode and be after Pac-Man again.

FRIGHTENED - Ghosts
enter frightened mode whenever Pac-Man eats one of the
four energizers located in the far corners of the maze. During
the early levels, the ghosts will all turn dark blue (meaning
they are vulnerable) and aimlessly wander the maze for a few
seconds. They will flash moments before returning to their
previous mode of behavior.

Reversal
Of Fortune

In
all three modes of behavior, the ghosts are prohibited from
reversing their direction of travel. As such, they can only choose
between continuing on their current course or turning off to one
side or the other at the next intersection. Thus, once a ghost
chooses which way to go at a maze intersection, it has no option
but to continue forward on that path until the next intersection
is reached.

Of course, if you've spent any time playing Pac-Man,
you already know the ghosts will reverse direction at
certain times. But how can this be if they are expressly
prohibited from doing so on their own? The answer is: when
changing modes, the system can override the ghosts' normal
behavior, forcing them to go the opposite way. Whenever this
happens, it is a visual indicator of their behavior changing from
one mode to another.

Ghosts are forced to reverse direction by the
system anytime the mode changes from: chase-to-scatter,
chase-to-frightened, scatter-to-chase, and scatter-to-frightened.
Ghosts do not reverse direction when changing back from frightened
to chase or scatter modes.

When the system
forces the ghosts to reverse course, they do not necessarily
change direction simultaneously; some ghosts may continue forward
for a fraction of a second before turning around.

The delay
between when the system signals a reversal and when a ghost
actually responds depends on how long it takes the ghost to enter
the next game tile along its present course after the reversal
signal is given (more on tiles in Chapter 3). Once the ghost
enters a new tile, it will obey the reversal signal and turn
around.

Scatter,
Chase, Repeat...

Ghosts alternate
between scatter and chase modes during gameplay at predetermined
intervals. These mode changes are easy to spot as the ghosts
simultaneously reverse direction when they occur. Scatter modes
happen four times per level before the ghosts stay in chase mode
indefinitely.

Good players will take full advantage of the scatter
periods by using the brief moment when the ghosts are not chasing
Pac-Man to clear dots from the more dangerous areas of the maze.
The scatter/chase timer gets reset whenever a life is lost or a
level is completed. At the start of a level or after losing a
life, ghosts emerge from the ghost pen already in the first of the
four scatter modes.

For
the first four levels, the first two scatter periods last for
seven seconds each. They change to five seconds each for level
five and beyond. The third scatter mode is always set to five
seconds. The fourth scatter period lasts for five seconds on level
one, but then is only 1/60th of a second for the rest
of play. When this occurs, it appears as a simple reversal of
direction by the ghosts.

The first and second chase periods last
for 20 seconds each. The third chase period is 20 seconds on level
one but then balloons to 1,033 seconds for levels two through
four, and 1,037 seconds for all levels beyond-lasting over
17 minutes! If the ghosts enter frightened mode, the scatter/chase
timer is paused.

When time runs out, they return to the mode they
were in before being frightened and the scatter/chase timer
resumes. This information is summarized in the following table
(all values are in seconds):

Mode

Level
1

Levels
2-4

Levels
5+

Scatter

7

7

5

Chase

20

20

20

Scatter

7

7

5

Chase

20

20

20

Scatter

5

5

5

Chase

20

1033

1037

Scatter

5

1/60

1/60

Chase

indefinite

indefinite

indefinite

Frightening
Behavior

Anytime Pac-Man
eats one of the four energizers on the level, the ghosts reverse
direction and, on earlier levels, go into frightened
mode. Frightened ghosts turn dark
blue and wander about the maze for a few moments, flashing briefly
as a warning before returning to normal. Ghosts use a
pseudo-random number generator (PRNG) to pick a way to turn at
each intersection when frightened.

The PRNG generates an
pseudo-random memory address to read the last few bits from. These
bits are translated into the direction a frightened ghost must
first try. If a wall blocks the chosen direction, the ghost then
attempts the remaining directions in this order: up,
left, down, and right,
until a passable direction is
found. The PRNG gets reset with an identical seed value every new
level and every new life, causing predictable results.

This is why
the frightened ghosts' movements are repeatable in the context of
a memorized pattern or sequence. As the levels progress, the time
ghosts spend in frightened mode grows shorter until eventually
they no longer turn blue at all (they still reverse direction).
Refer to Table
A.1 in the
appendices for the frightened time in seconds and number of
flashes, per level.

Speed

The game starts
with Pac-Man at 80% of his maximum speed. By the fifth level,
Pac-Man is moving at full speed and will continue to do so until
the 21st
level. At that point, he slows
back down to 90% and holds this speed for the remainder of the
game. Every time Pac-Man eats a regular dot, he stops moving for
one frame (1/60th
of a second), slowing his
progress by roughly ten percent-just enough for a following
ghost to overtake him.

Eating an energizer dot causes Pac-Man to
stop moving for three frames. The normal speed maintained by the
ghosts is a little slower than Pac-Man's until the 21st
level when they start moving
faster than he does. If a ghost enters a side tunnel, however, its
speed is cut nearly in half.

When frightened, ghosts move at a
much slower rate of speed than normal and, for levels one through
four, Pac-Man also speeds up. The table below summarizes the speed
data for both Pac-Man and the ghosts, per level. This information
is also contained in Table
A.1 in the
appendices.

PAC-MAN
SPEED

GHOST
SPEED

LEVEL

NORM

NORM
DOTS

FRIGHT

FRIGHT
DOTS

NORM

FRIGHT

TUNNEL

1

80%

71%

90%

79%

75%

50%

40%

2
- 4

90%

79%

95%

83%

85%

55%

45%

5
- 20

100%

87%

100%

87%

95%

60%

50%

21+

90%

79%

-

-

95%

-

50%

Cornering

Pac-Man is able to
navigate the turns in the maze faster than his enemies. He does
not have to wait until he reaches the middle of a turn to change
direction as the ghosts do (see picture below). Instead, he may
start turning several pixels before he reaches the center of a
turn and for several pixels after passing it.

Turns taken one or
more pixels before reaching the center are "pre-turns";
turns taken one or more pixels after are "post-turns".
Players learn to consistently move the joystick in the direction
Pac-Man should go well before arriving at the center of a turn,
ensuring each pre-turn is started as many pixels away from center
as possible.

This technique is known as cornering and is
one of the first skills a new Pac-Man player should master.
For every successful pre-turn maneuver, Pac-Man puts a little more
distance between himself and any ghosts following close behind.

Such a small gain in distance may not seem terribly significant at
first, but cornering through a quick series of turns will shake
off even the most determined pursuer. It is a vital tool for
survival in the higher levels of the game.

Whenever Pac-Man
makes a pre-turn or post-turn, his orientation changes, and he
starts to move one pixel in his new direction for every pixel
traveled in his old direction, effectively doubling his speed as
he moves at a 45 degree angle. Once he reaches the centerline of
the new direction's path, he starts moving purely in that
direction and his speed returns to normal.

The greatest distance
advantage is thereby gained by making the earliest pre-turn
possible. The illustration below shows the layout of pre-turn
pixels (shown in green), center point pixels (shown in yellow),
and post-turn pixels (shown in red) for each of the four possible
directions a turn can be approached. Each example shows Pac-Man
entering the same four-way intersection from a different
direction.

When entering from the left, there are three pre-turn
pixels before the center of the turn, and four post-turn pixels.
Conversely, entering the same intersection from the right yields
four pre-turn pixels and three post-turn ones. Entering from the
top as opposed to the bottom exhibits the same property.

For any
turn that is made later than the earliest possible pre-turn,
Pac-Man will be one frame behind where he would be for every pixel
of "lateness" in the turn. Basically, it pays to move
the joystick well before reaching a turn to maximize your speed.

(click image for full size)

Turning at the
earliest pre-turns possible is also required to successfully
execute most any pattern. Patterns are meant to be played with
perfect cornering because it removes the human element of
uncertainty as to when Pac-Man will turn. Without cornering, it
would be nigh-impossible to reproduce the exact timing of every
turn as made by the pattern's author, thereby increasing the
possibility of unpredictable ghost behavior due to Pac-Man not
being in the exact same tile at the exact same time anymore.

Typically, the most popular patterns have been those that tend to
"hold together" well when small input timing flaws
occur (turning three pixels away from center instead of four when
approaching a turn from the right is a timing flaw, for example).
Other patterns-especially those that bring Pac-Man very
close to the ghosts late in the sequence-tend to "fall
apart" unless every turn is perfectly cornered.

During a
long Pac-Man session, even
the best players will make occasional timing mistakes during a
fast series of turns and have to deal with the possible
consequences. As such, one should aim for perfect cornering at all
times but remain alert for unexpected ghost behavior from subtle
input timing flaws creeping into the pattern.

Home Sweet Home

Commonly
referred to as the ghost house or monster pen, this
cordoned-off area in the center of the maze is the domain of the
four ghosts and off-limits to Pac-Man.

Whenever a level is
completed or a life is lost, the ghosts are returned to their
starting positions in and around the ghost house before play
continues-Blinky is always located just above and outside,
while the other three are placed inside: Inky on the left, Pinky
in the middle, and Clyde on the right.

The pink door on top is
used by the ghosts to enter or exit the house. Once a ghost
leaves, however, it cannot reenter unless it is first captured by
Pac-Man-then the disembodied eyes can return home to be
revived. Since Blinky is already on the outside after a level is
completed or a life is lost, the only time he can get inside the
ghost house is after Pac-Man captures him, and he immediately
turns around to leave once revived.

That's about all there is to
know about Blinky's behavior in terms of the ghost house, but
determining when the other three ghosts leave home is an involved
process based on several variables and conditions. The rest of
this section will deal with them exclusively. Accordingly, any
mention of "the ghosts" below refers to Pinky, Inky,
and Clyde, but not Blinky.

The first control
used to evaluate when the ghosts leave home is a personal counter
each ghost retains for tracking the number of dots Pac-Man eats.
Each ghost's "dot counter" is reset to zero when a
level begins and can only be active when inside the ghost house,
but only one ghost's counter can be active at any given time
regardless of how many ghosts are inside.

The order of preference
for choosing which ghost's counter to activate is: Pinky, then
Inky, and then Clyde. For every dot Pac-Man eats, the preferred
ghost in the house (if any) gets its dot counter increased by one.
Each ghost also has a "dot limit" associated with his
counter, per level.

If the preferred ghost reaches or exceeds his
dot limit, it immediately exits the house and its dot counter is
deactivated (but not reset). The most-preferred ghost still
waiting inside the house (if any) activates its timer at this
point and begins counting dots.

Pinky's dot limit
is always set to zero, causing him to leave home immediately when
every level begins. For the first level, Inky has a limit of 30
dots, and Clyde has a limit of 60. This results in Pinky exiting
immediately which, in turn, activates Inky's dot counter. His
counter must then reach or exceed 30 dots before he can leave the
house.

Once Inky starts to leave, Clyde's counter (which is still
at zero) is activated and starts counting dots. When his counter
reaches or exceeds 60, he may exit. On the second level, Inky's
dot limit is changed from 30 to zero, while Clyde's is changed
from 60 to 50. Inky will exit the house as soon as the level
begins from now on.

Starting at level three, all the ghosts have a
dot limit of zero for the remainder of the game and will leave the
ghost house immediately at the start of every level.

Whenever a life is
lost, the system disables (but does not reset) the ghosts'
individual dot counters and uses a global dot counter instead.
This counter is enabled and reset to zero after a life is lost,
counting the number of dots eaten from that point forward.

The
three ghosts inside the house must wait for this special counter
to tell them when to leave. Pinky is released when the counter
value is equal to 7 and Inky is released when it equals 17. The
only way to deactivate the counter is for Clyde to be inside the
house when the counter equals 32; otherwise, it will keep counting
dots even after the ghost house is empty.

If Clyde is
present at the appropriate time, the global counter is reset to
zero and deactivated, and the ghosts' personal dot limits are
re-enabled and used as before for determining when to leave the
house (including Clyde who is still in the house at this time).

If dot counters
were the only control, Pac-Man could simply stop eating dots early
on and keep the ghosts trapped inside the house forever.
Consequently, a separate timer control was implemented to handle
this case by tracking the amount of time elapsed since Pac-Man has
last eaten a dot. This timer is always running but gets reset to
zero each time a dot is eaten.

Anytime Pac-Man avoids eating dots
long enough for the timer to reach its limit, the most-preferred
ghost waiting in the ghost house (if any) is forced to leave
immediately, and the timer is reset to zero. The same order of
preference described above is used by this control as well. The
game begins with an initial timer limit of four seconds, but
lowers to it to three seconds starting with level five.

The more astute
reader may have already noticed there is subtle flaw in this
system resulting in a way to keep Pinky, Inky, and Clyde inside
the ghost house for a very long time after eating them. The trick
involves having to sacrifice a life in order to reset and enable
the global dot counter, and then making sure Clyde exits the house
before that counter is equal to 32.

This is accomplished by
avoiding eating dots and waiting for the timer limit to force
Clyde out. Once Clyde is moving for the exit, start eating dots
again until at least 32 dots have been consumed since the life was
lost. Now head for an energizer and gobble up some ghosts. Blinky
will leave the house immediately as usual, but the other three
ghosts will remain "stuck" inside as long as Pac-Man
continues eating dots with sufficient frequency as not to trigger
the control timer.

Why does this happen? The key lies in how the
global dot counter works-it cannot be deactivated if Clyde
is outside the house when the counter has a value of 32. By
letting the timer force Clyde out before 32 dots are eaten, the
global dot counter will keep counting dots instead of deactivating
when it reaches 32. Now when the ghosts are eaten by Pac-Man and
return home, they will still be using the global dot counter to
determine when to leave.

As previously described, however, this
counter's logic only checks for three values: 7, 17, and 32, and
once those numbers are exceeded, the counter has no way to release
the ghosts associated with them. The only control left to release
the ghosts is the timer which can be easily avoided by eating a
dot every so often to reset it. Click on the YouTube video below
to see a demonstration of this curious behavior:

The last thing to
mention about the ghost house is how to determine whether a ghost
will move right or left after exiting the home. Ghosts typically
move to the left once they get outside, but if the system changes
modes one or more times when a ghost is inside, that ghost will
move to the right instead of the left upon leaving the house.

Areas To Exploit

The illustration
above highlights four special "zones" in the maze
where ghost behavior is limited by certain conditions which can be
exploited to the player's advantage. The two red zones represent
the areas where ghosts are forbidden to make upward turns. Once a
ghost enters either of these two zones, it may only travel from
right-to-left or left-to-right until exiting the area.

Thus, only
Pac-Man has access to these four, upward-facing tunnel entrances.
It will serve the player well to remember the ghosts can still
access these tunnels from the other end! The red zone restrictions
are enforced during both scatter and chase modes, but in
frightened mode the red zones are ignored temporarily, allowing
the ghosts to turn upwards if they so choose.

The pink zones are
in the two halves of the connecting side-tunnel. As mentioned
previously, any ghost that enters the tunnel will suffer an
immediate speed penalty until leaving the zone. This slow-down
rule is always enforced and applies to ghosts only-Pac-Man
is immune.

Chapter 3: Maze Logic 101

We need to take a
look at how ghosts are able to move through the maze in pursuit of
a goal. All pathfinding logic described in this chapter is shared
by the four ghosts - it is important to understand what they
have in common before we get into what makes them different.

Before we proceed, let's see how
the game tracks the location of Pac-Man and the four ghosts
(herein referred to as actors for
brevity's sake). The visible game screen should be thought of as a
regular grid of tiles, each eight pixels square.

The actual pixel
dimensions of the screen are 224 x 288, so dividing each value by
eight yields a grid that is 28 x 36 tiles in size.

Each tile is either
in legal space or dead space. In the picture above, legal space is
shown as the gray-colored tiles; all other tiles are considered
dead space.

Actors only travel between the tiles in legal space.
Each dot sits in the center of a tile, meaning they are exactly
eight pixels (one tile) apart-this is useful for estimating
distances during gameplay:

What Tile Am I In?

As the actors move
through the maze, the game keeps track of the tile each one
occupies. An actor is only associated with a single tile at a
time, although its graphic will overlap into the surrounding
tiles. The location of the actor's center point is what determines
the tile it occupies at any given time. As the actors can move at
pixel-level precision, they are often not centered directly on top
of the tile they are in. Consider the following example:

(click image for full size)

The transparent red
ghost is moving left-to-right across a row of tiles in legal
space. In frame one, its occupied tile (shown in bright red) is
near the left side of the picture. It does not matter that some of
the ghost's graphic is not in the tile-what matters is that
the ghost's center point is in the tile.

By frame two, it
has moved far enough for its center point to be in the adjacent
tile to the right and its occupied tile is updated accordingly.
The ghost continues to be associated with the same tile until
frame six where its center point has now crossed over into the
next one.

The underlying
concept of tiles is essential for understanding the ghosts'
pathfinding logic as it only cares about
the tile an actor occupies-not its per-pixel location within
that tile. To the logic routines, the five actors look very much
like the picture below.

Each actor is defined by the tile it
presently occupies along with its current direction of travel.
Distances between actors are also measured in tiles (the pink
ghost is five tiles away from Pac-Man horizontally and one tile
away vertically, for example).

Just Passing Through

It wasn't too long
after the release of Pac-Man when
word began to spread of players occasionally passing straight
through a ghost unharmed, seemingly at random. This rumor turned
out to be completely true as most die-hard Pac-Man players
can attest.

If you play the game long enough, you will eventually
see Pac-Man run into one of the ghosts and come out unscathed on
the other side-it doesn't happen very often so enjoy it when
it does! Some players have even gone so far as to incorporate this
mysterious pass-through oddity into their patterns.

The root cause of
this elusive peculiarity lies in the way the game detects
collisions between Pac-Man and the four ghosts. Any time Pac-Man
occupies the same tile as a ghost, he is considered to have
collided with that ghost and a life is lost.

It is irrelevant
whether the ghost moved into Pac-Man's tile or Pac-Man into the
ghost's-the result is the same either way. This logic proves
sufficient for handling collisions more than 99% of the time
during gameplay, but does not account for one very special case:

The above picture
illustrates the conditions necessary to produce this curious
behavior. There are five consecutive frames showing Blinky and
Pac-Man passing through each other. Below each frame is the same
scene represented by the tiles they currently occupy and the
per-pixel location of their center points. Pac-Man and Blinky are
at just the right position and speed relative to one another to
cause them to swap tiles with each other simultaneously.

In other
words, Pac-Man's center point moves upwards into Blinky's tile in
the same 1/60th of a second that Blinky's center point
moves downwards into Pac-Man's tile, resulting in them moving past
each other without colliding. Note that Pac-Man's origin point is
centered on the top edge of his tile in frame four; this is still
considered to be inside the bottom tile, but moving up one more
pixel will push him over the edge into the next one.

Pac-Man and
Blinky have now swapped tiles with each other in frame five, and
Pac-Man can go on his merry way because he never "collided"
(i.e., shared the same tile) with Blinky at all! Click on the
YouTube video below to see an example of the pass-through bug (it
happens 40 seconds after playback begins):

Target Tiles

Whenever a ghost
is in chase or scatter mode, it is trying to reach a target
tile somewhere on (or off) the screen. A target tile is merely
a way to describe the tile a ghost would like to occupy at any
given moment.

This tile can be fixed in place or change location
frequently. Whenever the ghosts
scatter to the corners of the maze, for example, each ghost is
striving to reach a fixed target tile located somewhere near its
home corner.

In chase mode, the target tile is usually (but not
always) related to Pac-Man's current tile which changes often.
Although it may not be obvious at first, the only difference
between chase and scatter mode to a ghost is where its target tile
is located. The same pathfinding logic applies in either case.

Looking
Ahead

Ghosts are always
thinking one step into the future as they move through the maze.
Whenever a ghost enters a new tile, it looks ahead to the next
tile along its current direction of travel and decides which way
it will go when it gets there.

When it eventually reaches that
tile, it will change its direction of travel to whatever it had
decided on a tile beforehand. The process is then repeated,
looking ahead into the next tile along its new direction of travel
and making its next decision on which way to go.

When a ghost looks
ahead into the upcoming tile, it must examine the possible exits
from that tile to determine a way to proceed. In the picture
below, the red ghost has just arrived at tile A and is moving
right-to-left. It immediately looks ahead to tile B (the next tile
along its direction of travel).

Each tile has four potential exits
to be considered: right, left, up, and down. In the case of tile
B, the up and down exits are blocked by walls and must be
discarded as potential candidates. The right exit is also
discounted because it would only take the ghost back to tile A
again, and ghosts never voluntarily reverse direction. With three
of the four possible exits eliminated from tile B, moving left is
the only remaining choice.

This example is the
most simple to explain as the ghost has but one way it can legally
move. As such, we did not have to worry about where its target
tile was located. The majority of game tiles in legal space are
similar to this one, but things get more interesting when a ghost
approaches a tile with more potential exits to choose from.

Intersections

When a ghost
arrives one tile away from an upcoming intersection, it must
choose between several possible directions in which to proceed.
Consider the following example:

(click image for full size)

In the first
picture, the red ghost has just reached tile A and is seeking its
target (shown as the green tile). It immediately looks ahead to
the subsequent tile along its present direction of travel (up). In
this case, that tile is a four-way intersection. As this
intersection tile has no walls blocking off any of the exits, the
ghost can only discard his reverse direction (down), leaving three
exits open for travel.

It looks one
tile beyond the intersection in each of the three remaining
directions, collecting "test tiles" (shown as
the tiles with dashed, white lines). In the middle picture, the
ghost triangulates the distance from each of these test tiles to
its target tile.

Whichever direction's test tile has the shortest
distance to the target becomes the direction the ghost will take
upon reaching the intersection tile. In this case, the right test
tile has the shortest distance to the target, and the ghost
updates its chosen direction for the intersection tile
accordingly.

Sometimes
a ghost is presented with two or more test tiles that have the
same distance to the target tile. In the example below, the red
ghost must choose between moving down or left at the upcoming
intersection tile. Unfortunately, both test tiles have the same
distance to the target (bottom left).

To break the tie, the ghost
prefers directions in this order: up,
left, down, right. Up
is the most preferred direction; right is the least. Therefore,
the ghost chooses to go left at the intersection because left
precedes down in the preference list. Although it may seem obvious
to a person that going down was the better choice to reach the
target, ghosts are not that smart. They cannot see more than a few
tiles ahead and, as a consequence, cannot recognize the disparity
between these two options.

Scatter Targets

As mentioned
before, each ghost has a fixed target tile it is trying to reach
in scatter mode. The picture below shows the actual tile used by
each ghost. Notice each target tile is in dead space on either the
top or bottom edge of the screen. As such, the ghosts will never
be able to reach them.

Luckily, a ghost does not care if its goal
is attainable or not-the A.I. routines are very
short-sighted. All a ghost cares about is following the
pathfinding logic described above to make the best choice it can
on which way to turn at the next tile.

As a result, it will simply
make circles in the area of the maze nearest its target tile until
the target is set to some other location. That's all scatter mode
really is. The only reason a ghost has a "favorite corner"
of the maze at all is due to the location of a target tile it will
never reach.

Chapter 4: Meet the Ghosts

"First,
you've got to learn how to control the monsters. See how the red,
pink and blue are grouped together? It's easier to control two
monsters than four."-Billy Mitchell, champion Pac-Man
player

In the last
chapter, we learned how a ghost follows a target tile through the
maze. Now we will take a closer look at Blinky, Pinky, Inky, and
Clyde to better understand why they behave so differently when in
chase mode. They all share the same pathfinding logic for chasing
a target tile, so how is it each one behaves differently when
following Pac-Man?

The answer is delightfully simple: Pac-Man's
tile is not always the target. Every ghost has a distinct
method for calculating its target tile in chase mode, resulting in
their unique personalities. Some of the ghosts use Pac-Man's
actual tile as the target; others only use it as an intermediate
step to find another tile.

Sometimes a ghost is targeting a tile
that has absolutely nothing to do with Pac-Man at all! Regardless
of where a ghost's target tile is at the time, Pac-Man will still
be killed if he gets in that ghost's way.

Rumor has it Toru
Iwatani and his team spent months doing nothing but tweaking and
refining the ghost A.I. routines before releasing Pac-Man to the
world. Their efforts show in the final product: Itawani's team
created the illusion of complex pathfinding by using very simple
logic and very little code.

Blinky:
The red ghost's character is aptly described as that of a shadow
and is best-known as "Blinky". In Japan, his
character is represented by the word oikake, which means
"to run down or pursue". Blinky seems to always
be the first of the ghosts to track Pac-Man down in the maze. He
is by far the most aggressive of the four and will doggedly pursue
Pac-Man once behind him.

Of all the ghosts'
targeting schemes for chase mode, Blinky's is the most simple and
direct, using Pac-Man's current tile as his target. In the
pictures above, we can see Blinky's target tile is the same as
Pac-Man's currently occupied tile. Targeting Pac-Man directly in
this way results in a very determined and tenacious ghost who is
tough to shake when he's right behind you.

All ghosts move at
the same rate of speed when a level begins, but Blinky will
increase his rate of speed twice each round based on the number of
dots remaining in the maze. While in this accelerated state,
Blinky is commonly called "Cruise Elroy", yet
no one seems to know where this custom was originated or what it
means.

On the first level, for example, Blinky becomes Elroy when
there are 20 dots remaining in the maze, accelerating to be at
least as fast as Pac-Man. More importantly, his scatter mode
behavior is also modified to target Pac-Man's tile in lieu of his
typical fixed target tile for any remaining scatter periods in the
level.

This causes Elroy to chase Pac-Man while the other three
ghosts continue to scatter as normal. As if that weren't bad
enough, when only 10 dots remain, Elroy speeds up again to
the point where he is now perceptibly faster than Pac-Man.

If a life is lost
any time after Blinky has become Elroy, he will revert back to his
normal behavior and speed when play resumes, heading for his home
corner during the initial scatter period. But once the last ghost
(Clyde) has left the ghost house in the middle of the board, he
will turn back into Elroy again.

Keep in mind: he is still
in scatter mode the
entire time. All that has changed is the target tile-he will
still reverse direction when entering and exiting scatter mode as
before. As the levels progress, Blinky will turn into Elroy with
more dots remaining in the maze than in previous rounds. Refer to
Table
A.1 in the
appendices for dot counts and speeds for both Elroy changes, per
level.

Pinky:
Nicknamed "Pinky", the pink ghost's character is
described as one who is speedy. In Japan, he is
characterized as machibuse, meaning "to perform an
ambush", perhaps because Pinky always seems to be able
to get ahead of you and cut you off when you least expect it.

He
always moves at the same speed as Inky and Clyde, however, which
suggests speedy is a poor translation of the more
appropriate machibuse. Pinky and Blinky often seem to be
working in concert to box Pac-Man in, leaving him with nowhere to
run.

In chase mode,
Pinky behaves as he does because he does not target Pac-Man's tile
directly. Instead, he selects an offset four tiles away from
Pac-Man in the direction Pac-Man is currently moving (with one
exception). The pictures below illustrate the four possible
offsets Pinky will use to determine his target tile based on
Pac-Man's orientation:

If Pac-Man is
moving left, Pinky's target tile will be four game tiles to the
left of Pac-Man's current tile. If Pac-Man is moving right,
Pinky's tile will be four tiles to the right. If Pac-Man is moving
down, Pinky's target is four tiles below.

Finally, if Pac-Man is
moving up, Pinky's target tile will be four tiles up and
four tiles to the left. This
interesting outcome is due to a subtle error in the logic code
that calculates Pinky's offset from Pac-Man. This piece of code
works properly for the other three cases but, when Pac-Man is
moving upwards, triggers an overflow bug that mistakenly includes
a left offset equal in distance to the expected up offset (we will
see this same issue in Inky's logic later).

Don Hodges' website
has an excellent article giving a thorough, code-level analysis of
this bug, including the source code and a proposed fix-click
here
to go there now.

Pinky is the
easiest ghost to exert control over thanks to his targeting
scheme. By changing direction, you can dictate where Pinky will
turn next when he is nearby (see above picture). If you are facing
off closely with Pinky, he will turn before he reaches you if he
can. This happens due to the fact Pac-Man has come close enough to
Pinky for Pinky's target tile to now be behind him.

In the picture above, Pinky chooses to turn up at the intersection because moving
left would have taken him further away from his target tile. The
longest-lived example of this is the technique known as "head
faking". This is where the player shakes the joystick to
cause Pac-Man to rapidly change direction back and forth,
hopefully causing a ghost to change course in the process.

As it
turns out, the shaking is not necessary-one well-timed,
quick reversal of direction towards Pinky just before he decides
what to do at an upcoming intersection is all that is needed to
get him off your tail.

Inky:
The light-blue ghost is nicknamed "Inky" and his
character is described as one who is bashful. In Japan, he
is portrayed as kimagure, meaning "a fickle,
moody, or uneven temper". Perhaps not surprisingly, Inky
is the least predictable of the ghosts.

Sometimes he chases
Pac-Man aggressively like Blinky; other times he jumps ahead of
Pac-Man as Pinky would. He might even wander off like Clyde on
occasion!

In fact, Inky may be the most dangerous ghost of all due
to his erratic behavior. Bashful is not a very good
translation of kimagure, and misleads the player to assume
Inky will shy away from Pac-Man when he gets close which is not
always the case.

Inky uses the most
complex targeting scheme of the four ghosts in chase mode. He
needs Pac-Man's current tile/orientation and Blinky's
current tile to calculate his final target. To envision Inky's
target, imagine an intermediate offset two tiles away from
Pac-Man's tile in the direction Pac-Man is moving (shown as the
dashed, green tile in the picture above), then draw a line from
Blinky's tile to that offset. Now double the line length by
extending the line out just as far again, and you will have Inky's
target tile as shown above.

For the same
reasons already discussed in Pinky's case, Inky's offset
calculation from Pac-Man is two tiles up and two tiles left
when Pac-Man is moving up (shown above). The other three
orientations have the expected offset of two tiles in the
direction Pac-Man is moving.

Inky's targeting
logic will keep him away from Pac-Man when Blinky is far away from
Pac-Man, but as Blinky draws closer, so will Inky's target tile.
This explains why Inky's behavior seems more variable as Pac-Man
moves away from Blinky. Like Pinky, Inky's course can often be
altered by Pac-Man changing direction or "head-faking".
How much or how little effect this will have on Inky's decisions
is directly related to where Blinky is at the time.

Clyde:
The orange ghost is nicknamed "Clyde" and is
characterized as one who is pokey. In Japan, his character
is described as otoboke, meaning "pretending
ignorance", and his nickname is "Guzuta",
meaning "one who lags behind".

In reality,
Clyde moves at the same speed as Inky and Pinky so his character
description is a bit misleading. Clyde is the last ghost to leave
the pen and tends to separate himself from the other ghosts by
shying away from Pac-Man and doing his own thing when he isn't
patrolling his corner of the maze.

Although not nearly as
dangerous as the other three ghosts, his behavior can seem
unpredictable at times and should still be considered a threat.

In
chase mode, Clyde's target differs based on his proximity to
Pac-Man. When more than eight tiles away, he uses Pac-Man's tile
as his target (shown as the yellow target above). If Clyde is
closer
than
eight tiles away, he switches to his scatter mode target instead,
and starts heading for his corner until he is far enough away to
target Pac-Man again.

In the picture above, Clyde is stuck in an
endless loop thanks to his targeting scheme. Outside of the dashed
area, Clyde acts exactly as Blinky would, heading straight for
Pac-Man, but upon entering the dashed area, Clyde will change his
mind and head for his scatter target instead.

Leaving the eight
tile perimeter surrounding Pac-Man causes his target to change
back
to
Pac-Man's tile and results in Clyde circling the island
indefinitely until Pac-Man moves elsewhere or a mode change
occurs.

Clyde's targeting
method results in him not being particularly dangerous unless you
get in his way as he runs back to his corner or before he can
reach an intersection to turn away. Extra care should be taken
when Pac-Man is in Clyde's home corner as Clyde is less likely to
get out of the way.

Chapter 5: Deciphering The Split Screen

"This is the way the world ends
This is the way the world ends
This is the way the world ends
Not with a bang but with a whimper."
—T. S. Eliot

On
The Edge Of Forever

Pac-Man was always
meant to be a game with no ending. The developers at Namco
mistakenly assumed the game's increasing difficulty was sufficient
to prevent anyone from playing indefinitely. Of course, within a
few years of Pac-Man's release, players had discovered that every
level beyond the 21st was identical.

Patterns were
quickly created to exploit this fact and, for any player able to
get past the first 20 levels, the game now became a test of
endurance to see how many points you could rack up before losing
focus and making a mistake. High scores soared into the millions
and most players agreed the game simply went on forever.

Eventually, a few highly-skilled players were able to complete 255
consecutive levels of play (scoring over three million points and
taking several hours to accomplish) and found a surprise waiting
for them on level 256. It was a surprise no one knew about-not
even the developers at Namco.

The
256th level displays the left half of the maze
correctly, but the right half is a jumbled mess of randomly
colored letters, numbers, and symbols. Notice the bonus counter in
the lower-right of the screen is also malfunctioning.

The left
side of the maze plays normally, but the right side is a different
story. Although both the player and the ghosts can navigate
through the right half of the screen, the original maze walls no
longer apply.

Instead, Pac-Man must be guided through a confusing
series of open areas, tunnels, one-way intersections, lone walls,
and pass-throughs-all invisible to the player-while
four ghosts are in hot pursuit.

Why does this
broken level happen in the first place? The culprit is the routine
responsible for drawing the bonus symbols along the bottom edge of
the screen. Here's what happens: when level 256 is reached, the
internal level counter is incremented to 255 (the level counter
starts at zero - not one) and the routine for drawing the
bonus symbols is called.

The routine loads the current level
counter value (255) into a CPU register and increments that
register by one. Unfortunately, 255 is the largest number that can
fit in a single byte which is the size of the Z-80 CPU registers,
so when the value is incremented the overflow is discarded leaving
a zero in the register instead of the expected value of 256. This
zero value leads the routine to believe this is an early level
since its value is less than seven.

The routine starts drawing
bonus symbols using the confused register as a counter. At the end
of every drawing loop, the register is decreased by one and then
checked to see if it is zero (the signal for the routine to stop
drawing symbols). Since the register already has a zero in it to
start, the first decrement will roll the value back to 255. It
will keep decrementing the register and drawing symbols until the
register is reduced to zero again, causing the loop to run a total
of 256 times.

This means that memory locations outside the bounds
of the bonus symbol table are drawn to the screen at increasing
locations in video memory. This half-broken level was named the
"split screen" by players; developers refer to it as a
"kill screen".

Playing
The Level

There
are 114 dots on the left half of the screen, nine dots on the
right, and one bonus key, totaling 6,310 points. When all of the
dots have been cleared, nothing happens. The game does not
consider a level to be completed until 244 dots have been eaten,
so there is nothing left to do but sacrifice Pac-Man to a hungry
ghost.

Interestingly, every time a life is lost, the nine dots on
the right half of the screen get reset and can be eaten again,
resulting in an additional 90 points per extra man. In the
best-case scenario (five extra men), 6,760 points is the maximum
score possible, but only 168 dots can be harvested-not
enough to change levels-so we are stuck. There are no more
dots to gobble or energizers to eat.

There is no final victory
waiting for Pac-Man, only an empty half-maze full of ghosts. The
game has an ending after all-just not a very happy or
exciting one.

Four of the nine
dots on the right half of the screen are invisible, but can be
heard when eaten. The picture on the left shows all nine dot
locations. Dots 1, 5, 6, and 9 are invisible; the rest can be seen
but some are a different color than normal.

Anyone reaching
this level quickly realized: to safely map out the right side of
the screen something had to be done about the ghosts. After much
tinkering, it was discovered that a ghost would get "trapped"
on the right edge of the screen if he got too close to it.

Once
trapped, a ghost can only move up or down but never right or left
again. By leading ghosts near the edge of the screen, a skilled
player could eventually get the ghosts out of the way and
concentrate on exploring the right half of the maze and collecting
the dots.

There
are many methods for trapping the ghosts. One of the easiest ways
to trap the three important ghosts is shown in the picture to the
right. The yellow line shows Pac-Man's path from the start of the
level to a spot near the bottom-right.

The exact instructions are
as follows: begin by going right until you reach a blue letter
'N', then go down. Keep going down until you reach a blue letter
'F', then go right. Keep going right until you reach a yellow 'B',
then go down again. When executed properly, Pac-Man will hit an
invisible wall almost immediately after the last turn is made. Now
we wait.

The red ghost will get stuck first. The pink ghost
follows a few seconds later. The blue ghost will continue to move
freely for several moments until the next scatter mode occurs. At
that point, it will try to reach some location near the right edge
of the screen and get stuck with the pink and red ghost instead.

Now the orange ghost is the only one still on the loose
(bottom-right). Clyde is no real threat, however, since he runs to
his corner whenever Pac-Man gets close (see Chapter 4), making it
relatively easy to clean up all the dots. Be sure to take care
around the lower-left corner of the maze-the orange ghost
will have nowhere left to run to and will be much more aggressive.

Click on the
YouTube video below to watch this ghost-trapping method in action:

Believe
It Or Not

Some versions of
the Pac-Man ROMs have a "rack test" feature,
allowing the cabinet owner to skip ahead to the next level of play
whenever they want. To date, the only known way to legitimately
get past level 256 is by using the rack test switch inside these
machines.

The result is that the game loops back around to the
first board, but with the score intact and the ghosts still
behaving as though it were level 21 or above. Many of the Pac-Man
ROMs available for use with the MAME emulator also have this rack
test feature, making it relatively easy for those without an
arcade version handy to quickly get to the split-screen and
beyond.

For decades,
Pac-Man enthusiasts worldwide have heard the whispers about
a "secret trick" allowing a player to get past level
256 and continue playing without using the aforementioned rack
test. Several players have boasted having acquired this holy grail
of Pac-Man knowledge over the years, but no one has been
able to make good on their claims by actually proving it.

This
topic became so hotly debated in the upper echelons of the arcade
gaming community that Billy Mitchell-who was convinced it
was impossible-offered a $100,000 cash prize to the first
player to prove they could legitimately get past level 256,
leaving the challenge open for a full year. The prize money went
unclaimed.

In spite of the
evidence against there being a way to get past level 256, rumors
still persist and can occasionally be found in classic gaming
forums online, yet no one has been able to back up their words
with indisputable proof. It's hard to imagine why anyone who could
legitimately get past the level did not collect Mr. Mitchell's
prize money to be sure.

Still the occasional whispers can be
heard. Perhaps it is simply natural for people to want to believe
in the possibility as opposed to not-like Santa Claus or the
Easter Bunny. Then again, maybe there is some middle-aged Pac-Man
junkie out there who is withholding secrets to a 30 year-old
amusement device for his or her own unfathomable reasons. Stranger
things have happened. You be the judge.

Appendix A: Reference Tables

(click image for full size)

Table
A.2 - Difficulty Specifications

There
is a small spot on the Pac-Man PCB where you can solder two pads
together to set the game to "hard" difficulty. The
only difference in hard difficulty is that five of the levels (1,
3, 6, 19, and 20) are eliminated from play. The system does not
eliminate any of the bonus symbols, however, causing much
confusion as to what level you're really playing.

Level two is the
first board in hard difficulty for example, but the cherry symbol
is used instead of the strawberry. It's still level two in terms
of gameplay, but with cherry symbols in place of the usual
strawberries. Also, the bonus point values are changed to match
the current symbol being used. You can determine the difficulty
setting of a machine by observing which ghost kills Pac-Man during
the attract mode demo game.

In normal difficulty, Pac-Man gets
captured by Inky in the lower-left area of the maze. If the
difficulty jumper has been connected, however, he is captured by
Clyde near the same location.

Normal

Normal
Bonus

Hard

Hard
Bonus

1

Cherries

-

-

2

Strawberry

2

Cherries

3

Peach
1

-

-

4

Peach
2

4

Strawberry

5

Apple
1

5

Peach
1

6

Apple
2

-

-

7

Grapes
1

7

Peach
2

8

Grapes
2

8

Apple
1

9

Galaxian
1

9

Apple
2

10

Galaxian
2

10

Grapes
1

11

Bell
1

11

Grapes
2

12

Bell
2

12

Galaxian
1

13

Key
1

13

Galaxian
2

14

Key
2

14

Bell
1

15

Key
3

15

Bell
2

16

Key
4

16

Key
1

17

Key
5

17

Key
2

18

Key
6

18

Key
3

19

Key
7

-

-

20

Key
8

-

-

21+

Key
9

21+

Key
4+

APPENDIX
B: Easter Eggs & Tricks

This
section is meant to contain not only the easter egg below, but
also tricks-interesting ways players have found to create
unexpected behavior in Pac-Man.
I have seen a few of these curious abuses of a Pac-Man
machine before, but
I don't know how to recreate them.

As such, I am asking for any
help the readers of this guide can provide towards expanding this
section. Please send any Pac-Man
tricks you may know
me for inclusion in the guide; full credit will be given for your
altruism and ingenuity.

NAMCO
Easter Egg

There
is a secret message hidden in Pac-Man by the developers at
Namco. To see it, put the machine into service mode and wait for
the settings screen to appear. Now quickly toggle service mode off
and on (an alignment grid will appear on the screen).

While
holding down the player 1 and player 2 buttons, toggle service
mode off and on again very quickly (if done properly, the
grid will stay on the screen), and then push the joystick in the
following directions: UP x 4, LEFT x 4, RIGHT x 4, DOWN x 4. The
message "MADE BY NAMCO" will appear sideways on the
screen, spelled out using energizers.

bonus symbol:
The often fruit-related symbol that appears twice per level below the ghost house and can be eaten for additional scoring. The point-value depends on the specific symbol and can range anywhere from 100 to 5,000 points each. Also known as fruit.

cherries:
The bonus symbol for the first round of play. Worth 100 points.

Clyde:
The orange ghost. Also known as “Guzuta” or “Mocky” in Puck-Man.

cornering:
The technique of moving the joystick in the direction one wishes to go well before reaching the center of a turn, ensuring Pac-Man will take the turn as quickly as possible.

Cruise Elroy:
When a certain number of dots are all that remain in a level, Blinky (red ghost) will change “gears”, speeding up as well as chasing Pac-Man even in scatter mode. He speeds up yet again when half the dots remain from the first change.

dots:
The 244 objects in the maze Pac-Man must eat to move on to the next round. There are 240 small dots worth 10 points each, and 4 energizer dots worth 50 points each. Also known as pills.

energizer:
One of four, large, flashing dots located near the corners of the maze worth 50 points each. When Pac-Man eats an energizer, the ghosts simultaneously reverse direction and, on earlier levels, turn dark blue. Pac-Man can then eat the blue ghosts for additional points, scoring more for each consecutive ghost eaten from one energizer: 200, 400, 800, and 1,600 points respectively. Also known as a power pills, fuel tanks, and vitamins.

flipping:
“Flipping the machine” refers to when a player earns one million points. The game is unable to display a score larger than 999,999, so the score readout “flips” over to zero and keeps counting. Also known as rolling the machine.

fruit:
See bonus symbol.

galaxian:
The bonus symbol for levels nine and ten. Also known as a tulip or a thunderbird. Worth 2,000 points.

ghost house:
The rectangular area in the middle of the maze where the ghosts start each new level and new life, returning to the house whenever they are captured by Pac-Man. Also known as the monster pen.

ghosts:
Pac-Man's four enemies in the maze are typically referred to as ghosts or monsters.

grapes:
The bonus symbol for levels seven and eight. Also known as a grenade. Worth 1,000 points.

grenade:
See grapes.

head faking:
Changing Pac-Man's direction back and forth in quick succession in an attempt to affect the turning logic of one or more ghosts in play. Blinky and Clyde do not use Pac-Man's current direction in their chase logic, so they are unaffected by head faking.

Inky:
The blue ghost. Also known as “Aosuke” or “Mucky” in Puck-Man.

intersection:
Anywhere pathways in the maze intersect, yielding more than one option on which way to proceed.

key:
The bonus symbol for levels 13 and above. Worth 5,000 points.

monster pen:
See ghost house.

monsters:
See ghosts.

orange:
See peach.

pattern:
A memorized series of turns associated with a particular level or levels that, when repeated, clears the maze of dots without getting Pac-Man captured by any of the ghosts. Also known as a routine.

peach:
The bonus symbol for levels three and four. Also known as an orange. Worth 500 points.

pills:
See dots.

Pinky:
The pink ghost. Also known as “Micky” in Puck-Man.

power pill:
See energizer.

routine:
See pattern.

side tunnel:
The connecting tunnel between the right and left edges of the screen. Entering this tunnel will “wrap” the player around to the other side of the screen. The monsters always suffer a speed penalty while in the tunnel while Pac-Man does not. Also known as The Tube, The Time Warp, and The Scoot.

split screen:
The 256th level of the game, where the right half of the screen is filled with garbage instead of the usual maze.

strawberry:
The bonus symbol for level two. Worth 300 points.

thunderbird:
See galaxian.

tulip:
See galaxian.

Frequently Asked Questions

Q:
How do I get past the split screen?

A:
The only known way to get past the split screen is via the “rack test” feature available on some Pac-Man ROMs (see Chapter 5).

Q:
What is a “perfect score”?

A:
A perfect score in Pac-Man is 3,333,360 points, which can only be attained by playing a perfect game. This requires catching all four ghosts at every energizer, gobbling down every bonus fruit, and never once losing a life for 256 consectutive levels of play.

All extra lives are needed once the split screen is reached to eat the nine dots hidden on the right side of the screen the maximum number of times—they respawn every time a life is lost. This was first achieved by Billy Mitchell of Hollywood, Florida in 1999.

Q:
Is it true that some of the ghost A.I. routines examine the joystick directly to make decisions?

A:
This is false. The memory-mapped IN0 joystick port is completely removed from the pathfinding and logic routines in the code.

Q:
What other games from the Pac-Man family will this ghost logic work with?

A:
Pac-Man Plus and Ms. Pac-Man both use the same basic pathfinding/targeting logic as the original Pac-Man. Many popular bootleg ROMs like the Atlantic City Chip and Hanglyman also use this logic.

Q:
Why are all of the bonus symbols food-related except for the galaxian, the bell, and the key?

A:
The galaxian was added as a nod to the Namco space-shooter title, Galaxian, which was under development at the same time as Pac-Man. No one knows why Toru Iwatani chose a bell and a key for the final two bonus symbols.

It has been theorized the bell may actually be some sort of food like an Asian cashew or even a blancmange dessert, which would make it consistent with the food theme. The bell at least has the possibility of being something else—no one has any theories on the key being anything but ... well ... a key.

Q:
Why are some members of the NAMCO development team listed by name but not others in Chapter 1?

A:
I have not been able to find out these persons' names—I'm sure they are documented somewhere but I have yet to find that information. If you know of any reliable sources for the names of the full NAMCO development team, please let me know ([email protected]).

[NOTE: The latest version of the Pac-Man Dossier is available at Jamey Pittman's website, and he can be contacted at [email protected] with questions, comments, and updates. Gamasutra will be working with Jamey on new dossiers for other games to be published in the future.]